The present invention relates generally to a method for recovering products from the defluorination of uranium hexafluoride. More particularly, the present invention relates to the recovery of commercial grade liquid anhydrous hydrogen fluoride and water insoluble stable triuranium octoxide.
Commercially useful uranium isotopes such as U235 have been produced in well known processes for over 40 years. The feed material for these processes have been produced from a uranium hexafluoride (UF6) enrichment process which takes natural uranium, which contains 0.7% U235, to suitable levels for nuclear fuel. The enrichment process leaves behind a UF6 material that contains mostly U238 and 0.1-0.3% U235. The material is referred to as depleted UF6 (DUF6) and as of yet has little commercial value although there is hope in the future that new enrichment technologies will allow for more of the U235 to be removed from the DUF6 essentially turning it into a resource.
The DUF6 that has been produced for the last 40 years is stored in carbon steel cylinders and amounts to around 50,000 cylinders or over 1 billion pounds of material. Storage of these cylinders is not considered a long term solution because of the potential for corrosion to the cylinders which could cause a release of the material into the environment. UF6 reacts readily with the moisture in the air to form hydrofluoric acid and water soluble uranyl fluoride (UO2F2). It is looked upon as a potential safety and environmental hazard. It is therefore desirable to have a cost effective process which can convert the DUF6 into its most stable insoluble form uranium oxide (U3O8). In addition, the process will create virtually no waste while recovering the hydrogen fluoride (HF) values of the DUF6.
While others have practiced the chemical equations mentioned in the present invention, (U.S. Pat. No. 5,346,648) no one has succeeded in putting together a complete process, or achieved the objectives of the present invention in the manner in which those objectives are achieved by the present invention. The present invention has fulfilled a long felt need for recovering commercially useful anhydrous hydrogen fluoride (AHF) from uranium hexafluoride (UF6). Furthermore the method of the present invention produces a stable insoluble uranium oxide, U3O8, that is less toxic than UF6 and can be stored for future use, disposed of in a low radiation level burial site at minimal cost, or used in current shielding applications.
While the method in U.S. Pat. No. 5,346,648 appears similar to the present invention, the present invention uses a liquid phase first reactor. Because of this it has numerous advantages over its predecessor. The present invention can be run at low temperatures and pressures. It can therefore use less expensive materials of construction than the super alloys required to withstand the high temperatures described in U.S. Pat. No. 5,346,648. The present invention is easier to control than its predecessor by nature of maintaining water in excess. The only feeds to the process are UF6 vapor and a small water makeup stream in the form of aqueous HF that is mixed into the internal recycle stream. It is safer to run than the method of U.S. Pat. No. 5,346,648 because it is run at low temperatures and pressures. Still another improvement is the intermediate uranyl fluoride hydrate that is made in present invention""s liquid phase reactor which is different than the uranyl fluoride intermediate made in its predecessor. Many other improvements exist and are realized.
The present invention is a method for recovering two distinct and separable products from the defluorination of uranium hexafluoride. The first product is a commercial grade liquid anhydrous hydrogen fluoride (AHF). The second is water insoluble uranium oxide such as uranium dioxide (UO2), uranium trioxide (UO3) and preferably, stable triuranium octoxide (U3O8) which can be stored safely for future use or disposed of in a conventional manner. A liquid recycle stream consisting of the azeotrope of water and hydrogen fluoride also exists and is used as a feed stock to the primary and or secondary reactor. The present method produces a commercially valuable material while reducing the amount of hazardous material that needs to be stored or disposed of in addition to making it less of a safety and environmental concern.
The method includes a primary reactor which is a reservoir, pump tank/settler/vaporizer around which a stream of an aqueous hydrogen fluoride solution circulates. A gaseous stream of uranium hexafluoride (UF6) is introduced into the circulating solution. The UF6 reacts with some of the excess water in the circulating stream producing a uranyl fluoride intermediate (UO2F2.H2O) and HF which dissolve in the solution. When the resulting solution has been saturated with UO2F2.H2O solid, the uranyl fluoride intermediate begins to precipitate out of solution and settles out at the bottom of the pump tank (settler). As water is reacted away and HF is evolved the resulting solution becomes more and more concentrated in HF and the resulting vapor is high in HF concentration. By controlling the temperature of the solution in the pump tank, HF rich vapor and water vapor essentially free of uranium can be condensed and fed into a conventional distillation column.
The solid uranyl fluoride intermediate produced is fed to a secondary reactor and reacted with water vapor to produce a uranium oxide product such as triuranium octoxide product and a gaseous mixture of water, hydrogen fluoride, and oxygen. This gaseous mixture is combined with the gaseous mixture of water and hydrogen fluoride from the primary reactor, condensed and subsequently fed into the conventional distillation column. The components are separated in a distillation column to obtain a commercial grade anhydrous hydrogen fluoride product stream overhead and an aqueous azeotropic recycle stream containing water and hydrogen fluoride. The azeotrope composition recycle stream is returned in part or in its entirety to the primary reactor as a water makeup to the system. The recycle stream can also be vaporized and combined with a small amount of makeup steam and used as a water feed source to the second reactor.
Hydrogen gas may be used in place of water as a feed to the secondary reactor with the resulting stream of HF and hydrogen
UO2F2.H2O+H2xe2x86x92U3O8+HF+H2
gas (small amount) being combined with the HF rich vapor from the primary reactor, condensed and fed into the conventional distillation column. In this case makeup water will need to be fed to the primary reactor.
A third reactor may be added that would act as a fluoride stripper for soluble fluorine in the triuranium octoxide product. Solid triuranium octoxide material is fed to the reactor and contacted with steam. If the third reactor is added any makeup water that is needed for the entire system is fed to it. The resulting mixture of steam with a very slight amount of HF is fed directly into the second reactor or should a hydrogen feed be selected as a reactant in the second reactor the steam and HF mixture would be condensed and fed to the primary reactor as makeup water.